plugin.php

<?php
/**
 * Plugin Name: Language Redirect
 */

add_action( 'init', 'language_redirect_init' ) {
    // Map language to blog id
    $sites = array(
        'default' => 1,
        'en' => 2,
        'de' => 2,
    );

    $lang = substr( $_SERVER['HTTP_ACCEPT_LANGUAGE'], 0, 2 );
    $id = array_key_exists( $lang, $sites ) ? $sites[ $lang ] : $sites['default'];

    if ( get_current_blog_id() != $id ) {
        wp_redirect( get_blogaddress_by_id( $id ) );
        die();
    }
}